7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Multi-fluid MHD studies of the magnetic flux ropes in un-magnetizedionospheres

1

Lianghai Xie1, Lou-Chuang Lee 2, Lei Li1
National Space Science Center, Chinese Academy of Sciences, Beijing 2 Institute of EarthSciences, Academia Sinica, Taiwan
e-mail (speaker): xielianghai@nssc.ac.cn

Magnetic flux ropes (MFRs) with twisted magnetic fieldlines are common in the solar system, most of which canbe caused by magnetic reconnection[1] or velocity shear[2].
However, these two mechanisms have difficulties in
explaining the formation of MFRs observed in theionosphere of an unmagnetized body, such as Mars,Venus, and Titan[3-4]. Here we propose a new mechanism:an azimuthal component of 𝛻 × 𝑬 can be generated by
the differential compression and diffusion at different
parts of the flux tube when the tube is sinking into thepartially ionized ionosphere, which generates azimuthal
magnetic fields with twisted magnetic field lines andfinally gives rise to an ionospheric MFR (Figure 1).

Figure 1. Schematic diagrams for the sinking andtwisting of flux tube. (a) A global view of flux tube
sinking, where the line segments of AB and A’B’
representing the tubes at times t0 and t’, with midpointsof O and O’, respectively. (b) The close view of the flux
tube at t=0 with (∇ × 𝑬)𝜙 = 0. (c) The (∇ × 𝑬)𝜙 canincrease when the tube is sinking, which produces 𝐵𝜙 .
The red arrows in (b) and (c) indicate the relative
magnitudes (not scaled) of the electric fields.
We develop a multi-fluid MHD model to study theformation of MFR in the ionospheres of Mars and Venus.
It is found that a straight flux tube can turn into a twisted
MFR during its sinking into the lower ionosphere,
caused by the differential compression at different partsof the tube (Figure 2). Further studies show that such a
MFR is not force-free structure, in which there is always
a pressure gradient force to balance the magnetic force.
Moreover, we find that the ionospheric ions inside theMFR can be squeezed out from the two ends of the MFR,
with an outflow flux of about 3.5 × 107 cm-2 s-1, which
can play an important role for the ion escape from theionosphere. In addition, we find that both the sinking
speed and the ratio of the ion gyroradius to the tuberadius can affect characteristics of the MFR. A MFR canbe more quickly formed for a larger sinking speed, but it
can be also more quickly filled by the ambient ions.
Oppositely, the MFR is more difficult to be filled when
the ratio of the ion gyroradius to the tube radius issmaller. Once the MFR get filled, the compression isprevented by an outward pressure gradient force, andboth the field strength and the rope helicity begin todecrease. Our results can explain the different altitudedependences of MFRs observed at Mars and Venus.

Figure 2. Simulation results of case 1 at t=1000 s. (a)-(e)show the results of Bz, By, 𝛼, Jz, and P in the y=0 plane,
respectively, where is α the helical pitch angle defined𝐵𝑦
as 𝛼 = 𝑎𝑡𝑎𝑛 . (g) shows the three-dimensional view
𝐵𝑧of magnetic field lines in z > 0 region, in which thedifferent colors on the field lines represent the magnitudeof Bz, the purple arrows indicate the field vectors
sampled along the line in the z=600 km plane, with adistance of 5 km from tube axis, and the purple lines in
the overlapped panels on the left indicate the hodogramsof the sampled magnetic fields.
References[1] Lee and Fu, Geophys. Res. Lett., 12, 105 (1985)[2] E. R. Priest, Astrophys. J., 344, 1010 (1989)[3] R. C. Elphic et al, J. Geophys. Res., 88, 2993 (1983)[4] H. Y. Wei et al, J. Geophys. Res., Icarus, 206 (2010)
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